Embodiments of the present invention relate generally to optical systems and assemblies, and more specifically to focusing methods for microscopic optical systems and assemblies.
A wide variety of microscopic optical systems exist that observe a sample of interest comprising biological or chemical substances. For example, sample imagers may be configured to detect activity that is indicative of a desired reaction (e.g., binding events between targets and probes). Such activity may be identified by detecting light emissions (e.g., fluorescence or chemiluminescence) from labels that are selectively bound to the targets or probes. The detected light is then analyzed to determine properties or characteristics of the biological or chemical substances. Other microscopic optical systems exist that are configured to inspect an object to determine certain features or structures of the object. For example, optical systems may be used to inspect a surface of a semiconductor chip or silicon wafer to determine whether there are any deviations or defects in a pattern on the surface. Other optical systems include profilometers that determine surface profiles of an object.
Conventional optical systems, such as those described above, generally include a focus-control system that determines whether the optical system has an acceptable degree-of-focus with respect to the object. For example, some conventional optical systems use a focusing method that includes reflecting a reference light beam off a surface of the object and detecting the reflected light beam with a detector (e.g., position-sensitive detector (PSD)). The reflected light beam forms a beam spot on a surface of the detector. If the beam spot is offset by a certain amount from a desired location on the surface or if the beam spot has a certain morphology (e.g., size, shape, and/or density), the focus-control system may determine that the optical system is not properly focused and may adjust the object or the optical components of the system accordingly.
However, the focus-control systems of such conventional optical systems have certain limitations. Focus-control systems often include several optical components that affect the optical path of the reference light beam before and after the light beam is reflected by the object. If any one of these optical components is somehow moved from a predetermined position during operation of the optical system or somehow adversely affected, the beam spot will not provide accurate information relating to the focus of the system. Such problems may not be identified until after an object is scanned thereby requiring the use of sub-standard data or possibly requiring another scan. In some cases, acquisition of another scan may not be possible and a valuable sample can end up being wasted. It may also be necessary to recalibrate the optical components of the focus-control system, which may take substantial time and costs to remedy. Sub-standard data, loss of samples, or time wasted in obtaining data can be particularly problematic in diagnostic or prognostic applications where samples are often scarce and the data provides information that is important in determining a course of treatment for a patient.
In addition to the above, conventional optical systems may use complex beam-spot analysis algorithms to analyze the location, shape, and density of the beam spot. Such analysis may be costly and also sensitive to the configuration of the optical components.
Accordingly, there is a need for focusing methods and focus-control systems that reduce the alignment sensitivity of the optical components. Furthermore, there is a need for focus-control systems that use alternative forms of beam-spot analysis. There is also a general need for improved focusing methods and focus-control systems that are simpler, more accurate, and/or less costly than known focusing methods and focus-control systems.